DESIGNING AND FABRICATION OF GASIFIER

Acknowledgement:

I would like to thank GHEaSES International Pvt. Ltd. and AEPC Nepal for giving me this platform to showcase my design ability. My heartfelt thanks to Dr. S. Varunkumar for his thesis on gasification. Special thanks to Rabin Dhakal for his continuous support and guidance. I would like to thank Er. Vishwa Prasanna Amatya, Er. Lal Babu Prasad and Er. Anu Shrestha for useful discussions and suggestions on the project. This project would not have been possible without their guidance and persistent help. I am also grateful to our team member Sirapa Shrestha and Asbina Baral. I extend a very special thanks to my brother, Er. Surya Prakash Gupta. I am also very thankful to all my friends of BME69 batch for their help and support during the project.

Introduction:

Gasifier is a high efficiency, low emission stoves, which promise constant power that can be controlled using any solid biomass fuel in the form of small wood chips or pellets. These stoves use battery run fan based air supply for gasification (primary air) and for combustion (secondary air). Design with the correct secondary air flow ensures combustion that allows attainment of peak combustion temperatures with accompanying high water boiling efficiencies (up to 50 % for vessels of practical relevance) and very low emissions (of carbon monoxide, particulate matter and oxides of nitrogen). The use of high density agro-residue based pellets or coconut shell pieces ensures operational duration of about an hour or more at power level of 3kWth(~12g/min).

Working Principle:
-The left side of the figure below is schematic view of a rice hull gasifier. Note that the air flows from the open top towards the bottom. The right side is a schematic of a reverse down-draft gasifier stove where the gasification air flows from bottom to top. In this reactor, shown on the left side of above figure, one can use biomass like wood chips, and pellets apart from ricehulls

-If now the reactor is operated such that air flows from the bottom and the fuel surface at the top is lit, one would get combustible gases that will burn above the the top surface with ambient air or with additional air supplied towards the top. Such a configuration, shown on the right side of Figure termed reverse downdraft gasifier constitutes the essence of a gasifier stove.
-An important consequence of this mode of operation is that the gas exiting from the top of the packed bed bears a fixed ratio to the amount of air introduced for gasification (primary air flowing from the bottom).
-The reduction reactions following the oxidation limit the amount of fuel to be consumed due to the endothermic nature of these reactions.
-The interesting feature is that the relative amounts of fuel consumed and air introduced remain the same and increased amount of solid is consumed when primary air flow rate increases. Thus the power of the stove is proportional to the primary air flow rate. The gases coming out of the bed will be at a temperature of 800 to 1100 K and will be composed of CO, H2, CH4, H2O (as gas), some higher hydrocarbons and N2. These gases are burnt to CO2 and H2O with a second stream of air which is introduced in the top region for this purpose.
-The biomass is largely wood chips and the forced convection depended on a fan; it formed the basis of camp stove
- Apart from rice hull the reactor can also be used for the thermo-chemical conver- sion of other biomass like pellets and wood chips. If in this reactor the gasification air is allowed to flow from bottom to top so that the generated gas, called producer gas, can be burnt on the top of the bed with a second stream of combustion air, the heat generated can be utilised for cooking applications.

Designing phase:

Designing the gasifier model was very difficult task. Lots of models are available in the internet. I first studied different models that were previously designed. During this I got to know many new things about it. In the same time I kept consulting with my teachers and seniors about it. I got different ideas and views from them. I firstly worked on the different available gasifier model. I designed a simple model in solidworks. Solidworks was a completely new software for me to work on it. I was in a learning phase. In the beginning it was a tough working in a solidworks. Tutorials available in youtube made me easy to learn different 3D tools of solidworks.

I first designed the forced convection type model which is a bit similar to “oorja jumbo” gasifier, a indian version of gasifier stove. But at that time I was puzzled about the materials to be used for obtaining the maximum efficiency and safety measures. For this I consulted with my teachers. They helped me a lot and provided me a good knowledge on selection of materials.

Model 1-Forced convection gasifier

After this design, I was worried about its adaptibity in rural areas. In many rural area of Nepal there is poor electrification and some of them still are completely deprived of electricity. Load shedding is also one of the major problem of Nepal. Model 1 require electricity for its complete operation. As rural people found model 1 expensive, I thought of designing another model which is less expensive and doesnot require any sort of external power for its operation. So, for this I used natural convection type model. This time i changed the design of model 1 from box type to semi hemispherical type. Its efficiency is less than the model 1 but it is more efficient than any other available stove in the market.

Model 2-Natural convection gasifier

Fabrication phase:

Fabrication was started after the complete design and analysis of different aspects of the gasifier. It was done in a group.

Group member:

Ravi Prakash Gupta

Yuvaraj Khatri

Shishir Humagain

Shrabin Rajbanshi

Arun Yadav

For fabrication part we used a metal sheet for its body parts.
-Firstly the base body is fabricated with the help of 2mm sheet metal in a rectangular shape as shown in figure.
-the body is then divided into 2 compartment upper and lower with the help thin sheet of 2mm, having hole of suitable diameter for placing the fuel grate, as shown in fig made of 3mm sheet metal

-the fuel grate is then welded on the thin sheet 

-the fuel container of ceramic refractory material or metallic cylinder with coated fire clay from inside of suitable diameter is then placed above the fuel grate

-the ash removal tray is just placed below the fuel grate on the lower compartment

-the outer supporting chamber of 0.5 to 1mm sheet metal is then fabricated as shown in figure
-the 2 suitable metallic valve of 1mm sheet metal is then fixed in the base body for controlling the air flow in both the compartment  During this phase we got knowledge to operate different machines like lathe machine, gilleting, milling, shaper, drilling, welding, rolling and so on. It was a fun working together in a group. We used to do our work in the leisure period.

        

Scope and future of gasifier in Nepal:

The gasification technology is new for Nepal. In spite of the technology being promising, the extent of work that has been carried out in this field was limited to laboratory experiment at research center for applied science and technology (RECAST) in early eighties. The Center for Energy Studies, Institute of Engineering. (CES, IOE) and many other institutions at present have been working in the field of gasifier stoves for both rural and commercial purpose.

NAST(Nepal Academy for Science and Technology) has developed a briquette gasifier stove by adopting the design of Asian Institute of Technology (AIT), Thailand. Students from bachelor of mechanical engineering, IOE, have also carried out successful research work in the field of institutional gasifier stove in association with NAST and CRE(Center for Renewable Energy) and AIT Thailand. However the government has not emphasized the technology so far. The tenth plan has mentioned that production of energy would be increased by applying various technologies used in organic energy like gasifier; cogeneration etc and necessary steps would be taken to rise in this regard.

The traditional stoves generally used have very high emissions and low conversion efficiency. The total efficiency of traditional cooking stove is only found about 10-12% on direct combustion .in the world over two billion people are using inefficient wood stoves, causing health problems and resulting in deforestation. Nepal is in frontline of the race. Electricity, gas or liquid fuels are preferred for cooking as they are clean and efficient, however the access to such energies is limited for the majority of the population which for the rural people to use fuel wood conventionally.

Conclusions:
In such a scenario, if the introduction of gasifier can be made in the indoor and outdoor application would be of great significance. The gasifier stove is more useful in cooking application in hostels, momo shops, hotels etc. it can reduce drudgery faced by the people of the country. Moreover it can also improve the health of the women who are primarily involved in cooking. Two classes of benefits are at the core of using gasifier stove: those internal to the household--money and time saved on acquiring fuel, reduced smoke in the home, and various conveniences in use--and those external to household s principally, diminished pressure on forest and energy resources and reduced greenhouse gases. The main direct beneficiaries of the programs are women and people in the middle-and lower-income levels of society. The gasifier can be made user friendly because of simple construction, low cost, easy operation, clean and fair heating.

Cheers,
Ravi Prakash
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